Osmosis is the net movement of water across a selectively permeable membrane when the two sides differ in dissolved-solute composition.

Consider two aqueous solutions separated by a membrane that water can cross but a dissolved solute cannot. Water molecules move randomly through the membrane in both directions, but the unequal solution compositions create a net thermodynamic tendency for water to move toward the side containing more nonpenetrating solute.

For dilute solutions with the same solvent, a useful introductory rule is:

net water movement is generally from lower concentration of nonpenetrating solute toward higher concentration of nonpenetrating solute.

The qualification matters because a solute that can itself cross the membrane may not sustain the concentration difference.

Example

Suppose pure water and a sucrose solution are separated by a membrane permeable to water but not sucrose. Water can cross in both directions, but the net movement is into the sucrose solution. If the receiving compartment can expand, its volume rises. If expansion is mechanically constrained, pressure builds until it can oppose further net water movement.

Osmosis is not solute diffusion

The sucrose does not need to cross for osmosis to occur. The defining transport is movement of water across a selectively permeable barrier.

If a solute can rapidly cross, its concentration difference may dissipate and the long-term effect on water movement can be very different. Membrane permeability must therefore be specified when predicting osmosis.

Aquaporin channels can greatly increase the rate at which water crosses a biological membrane without changing the direction favored by the existing osmotic driving force.

Osmosis is therefore a consequence of molecular diffusion plus selective permeability, not a special pump that pushes water toward solute.